Machine for producing solid moulded bodies, especially ice-cream bricks



0. G. HOYER 2,791,890 MACHINE FOR PRODUCING SOLID MOULDED BODIES, ESPECIALLY ICE-CREAM BRICKS 5 Sheets-Sheet 1 May 14, 1957 Fmled May 3. 1954 QLUF G HOYER Iuvam'oR BY MY. Punk ATTORNEYS May 14, 1957 0. s. HOYER 2,791,890

MACHINE FOR PRODUCING SOLID MOULDED BODIES, ESPECIALLY ICE-CREAM BRICKS Filed May 3, 1954 5 Sheets-Sheet 2 OLUF G. HOYER INVENTOR BY WMAMJMM PM ATTORN E Y5 0. G. HCYER MACHINE FOR PRODUCING SOLID BODIES, ESPECIALLY ICE-CREA May 14, 1957 MOULDED M BRICKS 5 Sheets-Sheet 5 Filed May 3. 1954 moans vs 2,791,890 MOULDED M BRICKS 5 Sheets-Sheet 4 2 a m Dunn. Wunu-Du v u H nun-DUE muuuuu hm y 4. 1957 o. s HC'JYER MACHINE FOR PRODUCING SOLID BODIES, ESPECIALLY ICE-UREA Filed May a, 1954 OLUF G. HOYER INVENTOR BY VM,Mq PM Arroauevs May 14, 1957 o O ER 2,791,890

MACHINE FOR PRODUCING SOLID MOULDED BODIES, ESPECIALLY ICE-CREAM BRICKS Filed May 3. 1954 5 Sheets-Sheet 5 OLUF G. HOYER INvENToR Arroanevs United States Patent MACHINE FOR PRODUCING SOLID MOULDED BODIES, ESPECIALLY ICE-CREAM BRICKS Oluf Gudmund Hiiyer, Aarhus, Denmark Application May 3, 1954, Serial No. 427,191 Claims priority, application Denmark May 11, 1953 16 Claims- (Cl. 62-114) The invention relates to a machine for producing solid moulded bodies, especially ice-cream bricks, by the refrigeration of liquid material, for example cream, which is poured into a mould with refrigeration cells, which during a circular motion around a stationary axis are alternately brought into contact with a refrigeration bath in a refrigeration zone and a thawing bath in a thawing zone.

For the continuous production of ice-cream bricks, machines have previously been proposed in which the refrigeration cells, where the cream is frozen, are conveyed in circular paths around a stationary axis, being during this movement alternately taken through a refrigeration bath and a thawing bath. These machines have the drawback that the refrigeration pockets should alternately be raised out of the refrigeration bath and lowered into the thawing bath and vice versa resulting in a complicated construction and operation.

With a view to remedying part of the drawbacks of the machines known of the said type, it has been proposed to let the two baths turn around said axis and participate in the movement of the refrigeration cells around the latter. This measure remedies part of the drawbacks, but at the same time requires that larger masses should be moved and likewise that larger masses should be alternately cooled and heated.

An object of the present invention is to provide a machine where the drawbacks of the known machines are avoided, but which nevertheless is of a simple construction.

According to the invention this is attained by the refrigeration bath being stationary and at the ends facing the thawing zone provided with openings, through which the refrigeration cells can pass into and out of the refrigeration zone. In an expedient embodiment, the ends of the tank containing the refrigeration bath comprises sluiceways with openings or channels which fit around the refrigeration cells passing through the sluiceways during the operation of the machine. Hereby a particularly simple construction is attained. The refrigeration cells are not lifted out of the refrigeration bath, but are moved around at the same level all the time, and in contradistinction to known machines, where this is also the case, a reduction of the masses to be moved and alternately cooled and heated during the operation is attained. The fact is that it is only the refrigeration cells with the carrying member, which together form the mould, that are to be moved, while the refrigeration bath with its surrounding insulation remains stationary. There is the further advantage that one has a free choice as regards the utilization of the mould area, which especially asserts itself when the machine is to be used alternately for the refrigeration of larger and smaller ice-cream bricks respectively, in which case the degree of filling of the form in the case of the smaller ice-cream bricks is higher than in the known machine, which owing to its construction is bound to a fixed number of refrigeration cells. In the 2,791,890 Patented May 14, 7

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2 known machines it is furthermore necessary to supply the refrigerant through a centrally located hollow column with the constructive complications deriving therefrom, among other things the necessity of using a bed plate on the floor.

The machine according to the invention can be used for producing moulded bodies of optional form, for example bars of ice-cream of triangular, oval, circular, rectangular, or any other polygonal cross section. The refrigeration cells may be conical or taper downwards in some other way. The liquid raw material, from which the moulded bodies are produced, may be optional, for example the material generally used for the production of edible ice-cream.

The channels in the sluiceways in the machine according to the invention expediently extend at least for part of the path of the refrigeration cells corresponding to the distance between two successive refrigeration cells. In this way the channels will always, irrespective of the position of the refrigeration cells, be closed by at least one row of refrigeration cells, and thus the quantity of refrigerant leaking out at the sluiceways will be a minimum or at any rate very slight.

in an advantageous embodiment of the machine according to the invention the refrigeration tank is formed by an upwardly open container of an open annular form, which container is of a substantially U-shaped cross section and has a perforated bottom which forms the ceiling in an underlying supply and distibution container, to which refrigerant is supplied through at least one supply pipe.

This construction is distinguished by its great simpiicity, there being required no large number of supply pipes, viz. one for each section of the segments provided in the refrigeration tank by dividing walls, as is the case with a known machine of the same type. The refrigeration tank is closed at its top by the mould which in a way known per se comprises an annular plate with upwardly open refrigeration cells suspended from the latter, which mould can be rotated in relation to the refrigeration tank, for example by being carried by a number of arms radiating from a rotating shaft.

In connection with the machine there are as is known per se, provided devices for filling cream into the refrigeration cells, for inserting carrying sticks into the moulded bodies produced in the refrigeration cells, when the said moulded bodies have obtained a suitable consistence, and for picking up the frozen moulded bodies by gripping the carrying sticks frozen into them. There may also be provided a device for coating the moulded bodies with chocolate.

The invention will now be further described with reference to the accompanying diagrammatical drawings, in which Fig. 1 shows an embodiment of a machine according to the invention, in side elevation,

Fig. 2 a plan view of the machine according to Fig. l with the parts located within the annular mould omitted,

Fig. 3 a diametral section through the machine in the immediate vicinity of the thawing zone,

Fig. 4 part of the machine in and around the thawing zone, and

Fig. 5 an evolved section along a circular line concentrically with the axis of rotation of the machine through the thawing zone.

The machine shown on the drawing is provided with an annular refrigeration mould 1 consisting of an annular upper plate, from which a large number of refrigeration cells 2 are suspended, which are arranged in radial rows and concentric circles. Along the periphery of the refrigeration mould there are, as appears from Fig. l, a filling apparatus 3 for filling the cells 2 with liquids or plastic raw material, a stick fitting device 4 for inserting sticks into the half-frozen moulded bodies, and a picking-up apparatus 5 which may be combined with a device for coating the moulded bodies with chocolate. Such a device has not been shown on the drawing.

The refrigeration mould 1, which is exchangeable, is mounted rotatable around a vertical shaft 36, Fig. 3, and is advanced intermittently by a mechanism, which will be further described in the following, in the direction indicated by the arrow A in Fig. 2. When the mould 1 stands still, the filling apparatus fills a radial row of cells 2 with the material to be frozen, and the mould is then advanced one section in the direction of the arrow, after which the next radial row, which is now in position under the filling apparatus, is filled, etc. The refrigeration starts at the filling, the cells under the filling apparatus being immersed in a flow of deep-cooled refrigerant or brine, e. g. a salt solution of calcium chloride cooled down to, for example, 25 C. in a refrigeration plant, not shown, located outside the machine. In the machine according to the invention the brine is, as will appear more clearly from the following, brought into contact with the cells in a special manner.

The filled cells 2 move stepwise and constantly washed by refrigerant towards the stick-fitting device 4. At the time when they reach the latter, a shell of the raw material has been frozen with a soft core of non-frozen material in the centre. The stick-fitting device automatically, during the period of immobility of the mould, inserts a carrying stick, generally a parallel-edged fiat wooden stick, into each core in the radial row of refrigeration cells 2 located in line with the stick-fitting device. The core not yet frozen has such a viscosity that it can keep the carrying stick in the position in which it is left by the stick-fitting device. From the stick-fitting device the cells travel on towards the picking-up apparatus 5 during constant refrigeration. At some distance before the picking-up apparatus 5, which distance may correspond to a few radial rows of cells, the refrigeration zone ends, and now follows first a neutral zone which is used for letting the refrigerant adhering to the cells drip off, and next a thawing zone in which the cells are surrounded by warm liquid for the purpose of loosening the blocks now frozen through, to enable the picking-up apparatus to pull them out of the cells, row by row.

The emptied cells 2 pass through still another neutral zone and then once more into the refrigeration zone, having now performed one complete revolution and being once more located under the filling apparatus 3, after which the process is repeated.

The picking-up apparatus has, as appears from Figs. 1 and 2, a number of radially disposed arms 6, in the example shown sixteen arms, each of which is provided with a number of tongs 7 corresponding to the number of cells in each radial row. In the example shown, this number is six. The arms are mounted on a picking-up head 8 stepping around a vertical axis, each step corresponding to the angular distance between two succesive arms. The stepping is synchronized with the turning of the refrigeration mould 1. In addition the arms perform an upward and downward motion. While the picking-up head stands still, only the two arms which at any time are in the positions designated by I and II participate in this motion. When the arm in the position I moves downwards, its tongs grip the carrying sticks in the underlying radial row of refrigeration cells, the frozen blocks contained in them having been loosened from the walls by passing through the thawing zone. The frozen blocks, which in Fig. l have been designated by 9, are lifted out of the cells and then hang freely in the air suspended from the tongs. The blocks next perform a circular motion from position I into position II. During the first part of this motion, a hardening of the surface of the blocks 9 occurs. Immediately after having been thawed loose, the surfaces of the blocks are wet, but in a comparatively short time the cold from the interior of the blocks penetrates to the surface and freeze it stiff. so that the blocks will then have dry surfaces. After this hardening the blocks can automatically be coated with chocolate.

The chocolate coating may be performed in a way known per se by means of a small container with molten chocolate moved up from below to immerse one radial row of blocks. Upon the immersion, i. c. after the chocolate container has been removed, there follows a hardening period for the chocolate coating, and when this has been accomplished, the treatment of the blocks in the machine is at an end. They have then reached position It. Here the blocks are subjected to the down ward motion previously mentioned, during which the Tongs open. The opening is effected by a little rod projecting from each tong actuating the closing mechanism by contact with a stationary abutment. The blocks drop down on to a belt conveyor, not shown, which leads them for example to an automatic packing machine.

While in the foregoing the ordinary method has been described a detailed account will now be given of the special features of the invention, particularly with reference to Figs. 3, 4, and 5.

The cooled refrigerant is supplied under a suitable pressure from a refrigeration plant, not shown, through a pipe 10 which is connected to the bottom of an annular supply and distribution container 13 with a perforated upper side 12, the openings of which are designated by M in the figure and which forms the bottom of the refrigeration tank 11 proper. In the example shown, the wall in the refrigeration tank 11 and the supply and distribution container 13 is in one piece, the separation of the two containers being provided by means of an inserted annular plate 12 forming an intermediate bottom.

The refrigerant rising through the supply pipe 10 makes its way into the supply and distribution container 13 and next through the holes 14 up into the refrigeration tank proper, from where it wells out over the high sides 16 of the refrigeration tank. The area of the cross section of the supply and distribution container 13 must of course be so large that an even distribution of refrigerant along the full extent of the refrigeration tank is obtained. The cells 2 of the intermittently rotating refrigeration mould 1, dip down into the refrigerant in the space 15 in the refrigeration tank 11 which is open at top. The refrigerant flowing over the edges of the side Walls 16 runs down into an accumulation tank shaped as an open annular ring with a cross section in the form of a trough 17 surrounding the refrigeration tank with the underlying supply and distribution con tainer. From the trough 17 the refrigerant returns to the refrigeration plant through a pipe 18. The trough 17 is enclosed by an insulation 19 which is externally protected by a sheathing 20.

The cells 2 enter and leave the space 15 of the refrigeration tank, the extent of which space in the direction of the motion corresponds to the extent of the refrigeration zone, through a pair of sluiceways 21 and 22. These sluiceways are incorporated as exchangeable, narrowed elongations of the two ends of the refrigeration tank. The sluiceways are exchangeable, so that, when the machine operates with cells of different dimensions. one may insert the sluiceway which fits the cells of the particular refrigeration mould as closely as permitted by manufacturing accuracy and operating conditions. The refrigerant leaving through the sluiceways will in a most effective way wash around the refrigeration cells. The quantity must, however, be kept down at a reason able value in order that there should be no risk of the level of the liquid in the refrigeration tank being reduced towards its ends. An inclination. to this may. however, be counteracted by the provision of a row of extra inflow holes 23 at the ends of the distribution container 13 facing the thawing zone, so that the inflow of liquid to the refrigeration container 15, which is open towards these ends, becomes somewhat larger here than at the middle. In this way it is possible to ensure a full liquid level right out to the beginning of the sluiceways.

The accumulation trough 17 is, as appears from Fig. 5, extended beyond the sluiceways 21 and 22 so that the whole of the outflowing quantity of liquid, apart from liquid adhering to the cells 2, is returned to the refrigeration plant.

A radial row of cells which leave the sluiceway 21, see Figs. 4 and 5, enter the thawing zone which at its ends has neutral zones where dripipng off can take place. The thawing zone is separated from the refrigeration zone by an insulated dividing wall 24 which belongs to the accumulation trough 17 and over which the cells can pass. The thawing is efiected by a container 25 with warm refrigerant being moved vertically up to immerse a number of radial adjacent rows of cells, in the case shown three such rows. This occurs while the mould 1 with the cells 2 stands still. When next the mould 1 is turned one section forwards, the container 25 will participate in the motion of the mould 1, and when the mould once more stops, the container 25 is again lowered and moves one section back, after which it is again moved upwards to enclose two of the rows it already enclosed once plus a new one. In this manner, each radial row of cells gets three immersions into the container 25, and during the third immersion they will be located in line with the picking-up apparatus 5, Figs. 1 and 2, and this apparatus pulls out the blocks from the cells.

The warm refrigerant is supplied to the container 25 by means of a circulation pump 26, Fig. 3, which draws liquid from the bottom of the container 27 and forces it to the container 25 through a hose 28 which permits the pump 26 and the container 25 to move in relation to each other. The liquid is heated by incorporated electric heating elements 29, the current supply to which is regulated by a thermostat, so that the temperature of the liquid is kept at a constant, but adjustable value. Liquid flowing over from the immersion container 25 will drop directly down into the container 27.

The container 25, the rectangular path of which is shown on the drawing by dotted lines in Fig. 5, is by means of a flange joint 30, Figs. 3 and 4, mounted at the end of a pipe 31 which through the hose 28 is connected to the circulation pump 26 for warm liquid. The pipe 31 is in fixed connection with a vertical rod 32 and is at right angles to the latter. The rod 32 can be slid up and down, running in four guiding rollers 33. These guiding rollers are mounted on a plate 34 which is in turn in fixed connection with a sleeve 35 which is j0ur nalled around the vertical shaft 36 by means of ball bearings 37. The free and closed end of the pipe 31 is led between two guides 38 so that the tube will always point towards the vertical axis of the machine. The vertical motion of the container 25 is produced by a cylinder 39 which may have either compressed air or oil as power-yielding medium. The reciprocating motion of the container is produced by the cylinder 40 which causes the container to swing around the vertical axis of the machine. The swinging motion is limited by adjustable stopping members 41 and 42. By this means, the reach of the motion can be adapted to the angular distance between two successive radial rows in the refrigeration mould.

The same mechanism as is used for producing the said motion of the container 25 may also be used for ad vancing the mould 1. With a view to this, a circular row of holes 43 have been punched in the upper plate of the mould 1, the centres of which holes lie at the same distance from the axis of the machine as the rod 32. During the upward motion of the container the rod can pass into one of the holes 43 which are provided in a number corresponding to the number of radial rows of cells in the mould 1. When the upward motion has been terminated and the container 25 is moved one section forwards by the cylinder 40, the mould 1 will be carried along due to the engagement of the rod 32 with one of the holes 43. Thus there will be no relative motion between the container 25 and the cells immersed in it, as long as the turning continues.

All other motions in the machine may be effected in a similar manner by means of cylinders of the same type as the cylinders 39 and 40. The supply of the driving medium to the difierent cylinders is controlled from a motor-driven, central battery of control valves. This consists substantially of a shaft with a number of cams which open and close the proper control valves in the proper succession, so that the different motions in the machine are co-ordinated in the correct manner. The cam can be driven from an electric motor with a constant rate of revolution through a continuously variable gear through which the rate of operation of the machine may be regulated.

The machine described is distinguished by operating fully-automatically and by having no open refrigerant surfaces.

It has already been mentioned that a better degree of filling is obtained in the case of the small sizes of cells 2 than is the case with the known machine. Fig. 2 shows a refrigeration mould 1 divided into twenty-four equal sections, each of which therefore has a range of 15. The limits of the sections are indicated by dot-and-dash lines, and, irrespective of the number of radial rows of cells in the mould, each of these lines forms center lines for a radial row of cells. There are, in other words, a whole number of radial rows of cells per section. If the three auxiliary devices, viz refrigeration machine, stick-fitting device, and picking-up apparatus, are placed at a distance from each other equal to a multiple of 15, these devices can be mounted so as to be stationary and will always be in a correct position irrespective of the number of radial rows in the mould. In the example shown, the pickingup apparatus and the filling apparatus are mounted at a distance from each other corresponding to two sections while there are six sections between the filling apparatus and the stick-fitting device.

Fig. 2 also shows three examples of the manner in which a section can be filled out with different sizes of ice-cream bricks. The three sections are designated by 50, 51, and 52, respectively, and contain six, seven, and eight radial rows, respectively. If, as is the case with the known constructions, a fixed distance between the rows were required, only six rows could be used in each section, irrespective of the size of the ice-cream bricks.

The invention is not limited to the embodiment example described. It is for example possible to use other numbers of radial and concentric rows of refrigeration cells and to use other forms of stick-fitting apparatuses and picking-up apparatuses. The degree of tightness between the refrigeration cells and the walls of the sluiceways depends on the construction of the machine. If rigid walls are used for the sluiceways, which will generally be the case, there must be a suitable clearance which prevents the cells and the walls from engaging each other. The tolerance is chosen in conformity with the other tolerances in the machine of importance for the paths of the cells through the sluiceways.

The advance of the mould can be effected in some other way, for example by means of a ratchet wheel mechanism, but the method indicated is particularly expedient, because each mould 1 can be provided with its row of holes corresponding to the distance it is to be advanced, so that in the case of an exchange of the refrigeration moulds the proper advance is automatically ensured.

By a suitable design of the end walls of the refrigeration tank, separate sluiceways can be dispensed with, the essential feature of the invention being first and foremost the avoidance of an up and down motion of cells which are to be conveyed into a stationary refrigeration bath. According to the invention, the cells are moved around in the same horizontal plane during the operation.

I claim:

1. Machine for the production of solidified moulded bodies by refrigeration of liquid material comprising a mould with refrigeration cells, means for conveying the mould in a horizontal path around a stationary axis, a stationary refrigeration bath forming a stationary rcfrig eration zone extending over part of said horizontal path, sluiceways terminating the refrigeration bath permitting level passage of the cells along said horizontal path and a thawing bath forming a thawing zone extending over another part of said horizontal path.

2. A machine as claimed in claim 1 in which said sluiceways are in the form of channels each extending over a distance corresponding to a distance between two successive refrigeration cells.

3. Machine as claimed in claim 1, wherein the refrigeration vessel is a stationary tank shaped as a part of an upwardly open annular ring of substantially U-shapcd cross section, and provided with a perforated bottom forming the ceiling in an underlying supplyand distribution-container, having at least one supply pipe for the refrigerant.

4. Machine as claimed in claim l, wherein the refrigeration vessel is a stationary tank shaped as a part of an upwardly open annular ring of substantially U-shaped cross section, and provided with a perforated bottom forming the ceiling in an underlying supplyand distribution-container having at least one supply pipe for the refrigerant, said tank being partly closed at the top by the mould comprising a part of an annular plate with upwardly open cells suspended from the latter.

5. Machine for the production of ice-cream bricks by refrigeration of soft raw material comprising a mould with cells, means for conveying the mould in a closed horizontal path around a vertical axis, a stationary refrigeration vessel in part of said path, end walls of the refrigeration vessel having openings therein permitting passage of the horizontally circulated cells without substantial loss of refrigerant, a thawing bath in another part of the path, a supplyand distribution-container having a perforated top which forms the bottom of the refrigeration vessel, and a heat insulated trough surrounding said vessel and said container and spaced therefrom.

6. Machine as claimed in claim 4 wherein the trough has at least one outlet for carrying away refrigerant welling over the walls of the refrigeration vessel.

7. A machine as claimed in claim 4 in which the top of the supply and distribution container has a greater number of perforations near said end walls than in the middle thereof.

8. Machine for the production of ice-cream bricks by refrigeration of soft raw material comprising a mould with cells. means for conveying the mould in a closed horizontal path around a vertical axis a stationary refrigeration vessel in part of said path. end walls of the refrigeration vessel having openings therein permitting level passage of the cells without substantial loss of refrigerant, :t thawing bath in another part of the path, a supplyund distribution-container having a perforated top which forms the bottom of the refrigeration vessel, a heat insulated trough surrounding said vessel and said container and spaced therefrom, said trough having an outlet for refrigerant, means for cooling said refrigerant and conveying it back to the refrigeration vessel.

9. Machine for the production of solidified moulded bodies by refrigeration of liquid material comprising a mould with refrigeration cells. means for conveying the mould in a path around an axis, a stationary refrigeration bath in a stationary refrigeration Zone extending over part of said path, sluiceways at the ends of the refrigeration bath permitting passage of the cells a thawing container in a thawing zone extending over another part of said path and means for moving the thawing container in a substantially rectangular path upwards, forwards, downwards, backwards, upwards, and so on.

10. Machine as claimed in claim 7 wherein the means for moving the thawing container comprises a carrying arm, guides guiding the end of the arm in said rectangular path, driving means for the upward and downward motion and driving means for the backward and forward motion along paths each being part of a circle around the axis of the refrigeration vessel.

ll. Machine as claimed in claim 7 wherein the means for moving the thawing container comprises a carrying arm, guides guiding the end of the arm in said rectangular path. a driving piston in a cylinder for the upward and downward motion and a driving piston in a cylinder for the backward and forward motion along paths each being part. of a circle around the axis of the refrigeration vessel.

12. Machine for the production of ice-cream bricks by refrigeration of soft raw material comprising a mould with cells, means for conveying the mould in a closed circular path, a stationary refrigeration vessel in part of said path, openings at the end walls of the refrigeration vessel permitting passage of the cells without substantial loss of refrigerant and a thawing container in another part of the path, said thawing container being carried by an arm, guides guiding the end of the arm in said rectangular path, a driving piston in a cylinder for the upward and downward motion, a driving piston in u cylinder for the backward and forward motion along paths each being part of a circle around the axis of the refrigeration vessel, and means controlled by said pistons for effecting a stepwise turning motion of the mould.

13. Machine as claimed in claim 10 wherein the means for stepwise moving of the mould comprises a driving rod, a row of meshing holes in the mould for subsequently receiving of the driving rod, and an actuating mechanism making the driving rod contact a hole during the upward motion of the thawing container and turning one step around the axis simultaneously with the forward motion of the thawing bath.

14. Machine as claimed in claim .10, wherein the means for stepwise moving of the mould comprises a driving rod connected with the said carrying arm, a row of meshing holes in the mould for subsequently receiving of the driv lug rod when moved in the rectangular path corresponding to that of the thawing bath.

15. Machine for the production of bodies of solidified ice-cream by refrigeration comprising a cell mould mounted horizontally rotatable around a vertical axis, a stationary refrigeration vessel in part of the circular path of the mould and of U-shapcd cross section openings at the end of the walls of the said vessel permitting passage of the cells without substantial loss of refrigerant, a thawing container combined driving means for moving the thawing container in a rectangular path and moving the mould stepwise forward during the forward stroke of the thawing container.

16. Machine as claimed in claim 13 wherein the driving means comprise a ringshaped, upper cell carrying plate having a circular arch of evenly distributed holes and a forwarding member in fixed connection with the thawing container.

References Cited in the file of this patent UNITED STATES PATENTS 1,924,988 Grayson Aug. 29, 1933 1,939,109 Davidson Dec. 12, 1933 1,939,334 Burke Dec. l2, 1.933 2,246,941 Hoyer June 24, 1941 2,332,367 Birdseye .c Oct. 19, 1943 (Other references on following page) 9 UNITED STATES PATENTS Reagin May 25, 1948 Van Vleck Apr. 12, 1949 Comstock Dec. 28, 1954 Gram Jan. 25, 1955 5 FOREIGN PATENTS Denmark Mar. 9, 1936 Netherlands Sept. 15, 1950 France Dec. 26, 1935 France Feb. 7, 1951 

